This invention relates to ring laser gyroscopes, and is concerned with the removal or compensation of errors which result when a bias or so-called dither is imparted to the gyroscope in order to reduce or avoid a phenomenon known as "lock-in".
As is well known, a ring laser employs two beams of light which are propagated in opposite directions around the ring or so-called cavity. The cavity is typically of triangular or rectangular form and may be of a modular construction or an integral construction. In a modular ring laser, one limb or part thereof is formed by a laser which produces the contra-propagating beams of light which are transmitted to the remainder of the ring through Brewster angle windows. In an integral cavity, the limbs of the ring communicate directly with each other and the ring is filled with a lasing medium which produces the desired contra-propagating beams of light.
In an ideal ring laser, the frequency difference between the beams of light is zero when the ring is stationary but moves from zero when the ring is rotated about a sensitive axis extending at right angles to the plane of the ring. Thus, the frequency difference between the two contra-propagating beams of light is proportional to the angular rotation rate of the ring about the sensitive axis. Therefore, a ring laser is capable of functioning as a rate gyroscope. In practical ring lasers, however, there are many defects which degrade the performance and the majority of these are linked in some way to the amount of light that is lost in the contra-propagating beams of light traversing the ring. One of the most dominant, and hence troublesome, effects is lock-in which is caused by light scattered from each beam interacting with the opposite beam, thereby suppressing the frequency difference at low rotation rates, and making the frequency difference a non-linear function at higher rates.
In a ring laser gyroscope, the contra-propagating beams of light are directed around the ring by mirrors disposed at the transitions between adjacent limbs of the ring. Normally one of these mirrors is partially transmissive, whereby part of each beam passes therethrough to provide an output light beam, the two output light beams being combined to provide interference fringes which may be counted by a photo detector. The fringe count is directly proportional to the total angle through which the ring laser has turned, provided the two beams of light are completely uncoupled. The ratio of the fringe count per unit angle of rotation is known as the scale factor. As a result of lock-in, no fringes will occur up to the lock-in threshold and the scale factor will be non-linear for a range of rotational rates above the lock-in threshold. Both these phenomena seriously degrade the accuracy of the ring laser gyroscope at low angular rates.
One method of avoiding the lock-in problem is to impart a bias to the ring laser such that a non-reciprocal phase shift is introduced into the two contra-propagating light beams. Various biassing or dithering techniques have been proposed, including oscillating the entire ring laser at a small amplitude. With the latter arrangement it is preferable to employ some means for introducing a random phase shift of the beams to prevent systematic errors accumulating from the oscillation; perturbation of the dither is a convenient way of introducing this random component.
This method has many advantages. Specifically, it is capable of yielding ring laser gyroscopes with very stable low bias. However, it has the disadvantage of inducing a random noise component into the output of the ring laser gyroscope which can be a severe limitation in applications where the use of the ring laser gyroscope is of short duration. Such applications include the gyrocompassing alignment of an aircraft navigation system prior to take-off, and the guidance of missiles.
The noise is also a disadvantage in that it increases gyroscope test times and it reduces the advantages of the electro-optical techniques which are available for increasing the resolution of a ring laser gyroscope.
An aim of this invention is substantially to reduce the output noise of dithered ring laser gyroscopes, thereby increasing their short term accuracy.